Read somewhere that an LED button can draw 25mA, and with 20 buttons that's 500mA. Plus the 150mA from the Pi I should be good to go. Thanks for the help.

The current through an LED depends on how bright you want it, what colour it is (red ones have lower forward voltage than green ones, blue are higher again) and what resistor you put in series with it.

If you want it nice and bright, let it have as much current as it can take. The spec. sheets for many LEDs suggest 20mA as maximum, but you'll have to look it up for the ones you have.

Alternatively, you could reduce the current to almost nothing, and see if it's still visible - on a dark moonless night, less than a mA could make it as bright as Jupiter.

As for the switch bit - an LED button is just a switch with a built-in LED - a 10kΩ pull-up to 3v3 would take 1/3 mA when it's pressed.